3G Evolution 15 Chapte r: LTE Radio Interface Architecture Department of Electrical and Information Technology Johan Löfgren 2009-03-19 3G Evolution - HSPA and LTE for Mobile Broadband 1
Outline • Architecture Basics • Protocol Architecture Overview • Short Description of Layers • Longer Descriptions – Radio Link Control (RLC) – Medium Access Control (MAC) – Physical Layer (PHY) • Channels and Channel Mapping – Logical Channels – Transport Channels – Physical Channels – Channel Mapping • Data Flow Example • Chapter Summary 2009-03-19 3G Evolution - HSPA and LTE for Mobile Broadband 2
Architecture Basics • LTE is strucutred into different protocol layers – Lower protocol layers provide services for upper layers • The data from/to higher layers is known as a Service Data Unit (SDU) • The data from/to lower layers is known as a Protocol Data Unit (PDU) – Each layer is responsible for different tasks • The upper layers should not have to care about the details • This structure is common to most modern systems – Some layers are almost universal – Others are LTE specific • In LTE, IP packets enters through on of the SAE bearers – SAE = System Architecture Evolution – The IP packets are then processed by the protocols 2009-03-19 3G Evolution - HSPA and LTE for Mobile Broadband 3
Protocol Architecture Overview 2009-03-19 3G Evolution - HSPA and LTE for Mobile Broadband 4
Short Description of Layers (1/2) • Packet Data Convergence Protocol (PDCP) – Performs IP header compression • Reduces the number of bits to transmit over the radio interfaced • Based on Robust Header Compression (ROHC) • Radio Link Control (RLC) – Responsible for • Segmentation/concatenation • Retransmission handling • In-sequence delivery to higher layers – Located in the eNodeB since no higher layers exists in LTE • In WCDMA this was handled higher in hierarchy 2009-03-19 3G Evolution - HSPA and LTE for Mobile Broadband 5
Short Description of Layers (2/2) • Medium Access Control (MAC) – Responsible for • Uplink/downlink scheduling • Hybrid-ARQ retransmissions • Choice of modulation • Resoucre assignment • Physical Layer (PHY) – Responsible for • Coding/decoding • Modulation/demodulation • Resource mapping 2009-03-19 3G Evolution - HSPA and LTE for Mobile Broadband 6
Radio Link Control (RLC) • The RLC splits and concatenate the incoming data to fit the system – The RLC PDU sizes are dynamically varied to fit the amount of data • Much data = long RLC PDUs to reduce RLC header overhead • Little data = short RLC PDUs to avoid too much padding • Responsible for error-free delivery to higher layers – Accomplished with retransmission protocol – Re-sequencing of data may be requiered – Complementary to Hybrid-ARQ in lower layers 2009-03-19 3G Evolution - HSPA and LTE for Mobile Broadband 7
Medium Access Control (MAC) (1/2) • Scheduling – The basic operation is so-called dynamic scheduling • Downlink/Uplink are independently schedueled • Time-frequency resources dynamically shared between users • Each 1 ms TTI a new decision is taken • Each mobile unit scheduled by the eNodeB – However each mobile still decides what radio bearer(s) to transmit – Scheduling strategy is not specified by 3GPP • Implementation specific • Normally aiming at taking advantage of channel variations • Channel status report from mobile unit to eNodeB 2009-03-19 3G Evolution - HSPA and LTE for Mobile Broadband 8
Medium Access Control (MAC) (2/2) • Hybrid ARQ with soft combining controlled by MAC – Different simultaneous processes running – When error in one part, only that process needs to retransmit – Soft combining with adaptive coding leads to better chance of reception 2009-03-19 3G Evolution - HSPA and LTE for Mobile Broadband 9
Physical Layer (PHY) • Responsible for – Coding – PHY Hybrid ARQ processing – Modulation • Controlled by MAC – Transport Format (TF) • Tells PHY modulation et c. Simplified PHY processing 2009-03-19 3G Evolution - HSPA and LTE for Mobile Broadband 10
Logical Channels • The MAC offers services to the RLC as logical channels – Broadcast Control Channel (BCCH) • System information to all mobile terminals – Paging Control Channel (PCCH) • Paging information when searching a unit – Common Control Channel (CCCH) • Random access information (setting up a connection) – Dedicated Control Channel (DCCH) • User-specific control information (power/handover et c) – Multicast Control Channel (MCCH) • Information needed for multicast reception – Dedicated Traffic Channel (DTCH) • Transmission of user data – Multicast Traffic Channel (MTCH) • Transmission of multicast data 2009-03-19 3G Evolution - HSPA and LTE for Mobile Broadband 11
Transport Channels • The PHY offers services to the MAC as transport channels – Broadcast Channel (BCH) • Maps to BCCH – Paging Channel (PCH) • To convey the PCCH – Downlink Shared Channel (DL-SCH) • Main channel for downlink data transfer, used by many logical channels – Multicast Channel (MCH) • Used to transmit MCCh information to set up multicast transmission – Upink Shared Channel (UL-SCH) • Main channel for uplink data transfer, used by many logical channels 2009-03-19 3G Evolution - HSPA and LTE for Mobile Broadband 12
Physical Channels • The PHY transmit the data over the physical channels – Physical Downlink Shared Channel (PDSCH) • Used for unicast transmission and paging – Physical Broadcast Channel (PBCH) • Carries system information for accessing the network – Physical Multicast Channel (PMCH) • Carries system information for multicast – Physical Downlink Control Channel (PDCCH) • Carries mainly scheduling information – Physical Hybrid ARQ Indicator Channel (PHICH) • Reports Hybrid ARQ status – Physical Control Format Indicator Channel (PCFICH) • Information to enable the terminalse to decode the PDSCH – Physical Uplink Shared Channel (PUSCH) • Uplink counterpart of PDSCH – Physical Uplink Control Channel (PUCCH) • Sends Hybrid ARQ acknowledgement – Physical Random Access Channel (PRACH) • Used for random access 2009-03-19 3G Evolution - HSPA and LTE for Mobile Broadband 13
Channel Mapping Downlink Channel Mapping Uplink Channel Mapping 2009-03-19 3G Evolution - HSPA and LTE for Mobile Broadband 14
Data Flow Example 2009-03-19 3G Evolution - HSPA and LTE for Mobile Broadband 15
Chapter Summary • LTE is structured into different protocol layers – RLC is responsible for splitting and concatenation of data – MAC controls data and Hybrid ARQ – PHY is responsible for actual transmission • Different Channels are interfaces to higher layers – Logical Channels – Transport Channels – Physical Channels 2009-03-19 3G Evolution - HSPA and LTE for Mobile Broadband 16
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